The present invention relates to a novel structure and fabrication process for a Schottky barrier field effect MESFET transistor. More particularly, the present invention relates to a structure and fabrication process for such a MESFET transistor in which the gate resistance is negligible so as to improve microwave operation.
A typical MESFET (Metal Semiconductor Field Effect Transistor) includes a semiconductor substrate, a source electrode ("S"), a gate electrode ("G") and a drain electrode ("D"). For MESFET structures used in the microwave field, a radio frequency ("RF") signal applied to the gate electrode G results in an aplified output signal on the drain electrode D. The source electrode is generally grounded. For MESFET structures operated in the microwave range, the electrical resistance of the gate electrode has a large bearing on performance of the MESFET structures. A low gate electrode resistance is important for achieving high power and low noise in MESFET structures operated at very high (e.g. microwave) frequencies in which the dimensions of the gate electrode are typically below a micron. The foregoing restriction on gate width normally results in a large gate electrode resistance in prior art devices.
The approaches for reducing gate electrode resistance in prior art MESFET devices can be basically classified as follows:
(1) Increasing the thickness of the metallization of the gate electrodes; or
(2) Decreasing the length of the individual arms of the gate electrode which, in a conventional device, comprise the individual arms of a "U"-shaped electrode.
As far as approach (1) is concerned, the gate electrode can only be increased in thickness to not more than 1 or 2 microns, due to various technological limitations. As to approach (2), reducing the length of the individual arms of the gate electrodes results in disadvantageously increasing the number of interconnections required per unit area of the device, which reduces manufacturing yield, and increasing the semiconductor areea required for each MESFET device, particularly for power devices, which results in degraded device performance, especially at high frequencies.